1. Field of the Invention
This present invention relates to a downhole tool for wireline and measurement-while-drilling applications, and in particular relates to a method and apparatus for cooling of electronic components deployed in a downhole tool suspended from a wireline or a drillstring.
2. Background of the Invention
In underground drilling applications, such as oil and gas exploration and development, a borehole is drilled through a formation deep in the earth. Such boreholes are drilled or formed by a drillbit connected to an end of a series of sections of drill pipe, so as to form an assembly commonly referred to as a “drillstring.” The drillstring extends from the Earth's surface to the bottom of the bore hole. As the drillbit rotates, it advances into the earth, thereby forming the borehole. In order to lubricate the drill bit and flush cuttings from its path as it advances, a high pressure fluid, referred to as “drilling mud,” is directed through an internal passage in the drillstring and out through the drill bit. The drilling mud then flows to the surface through an annular passage formed between the exterior of the drillstring and the surface of the bore.
The distal or bottom end of the drillstring, which includes the drillbit, is referred to as a bottomhole assembly (BHA). In addition to the drillbit, the BHA often includes specialized modules or tools within the drillstring that make up the electrical system for the drillstring. Such modules often include sensing modules, a control module and a pulser module. In many applications, the sensing modules provide the drillstring operator with information regarding the formation as it is being drilled through, using techniques commonly referred to as “measurement while drilling” (MWD) or “logging while drilling” (LWD). For example, resistivity sensors may be used to transmit and receive high frequency signals (e.g., electromagnetic waves) that travel through the formation surrounding the sensor. It is to be noted that the sensors fall within the dictionary definition of electronics:                “of, relating to, or utilizing devices constructed or working by the methods or principles of electronics; also: implemented on or by means of a computer.”        
In other applications, sensing modules are utilized to provide data concerning the direction of the drilling and can be used, for example, to control the direction of a steerable drillbit as it advances. Steering sensors may include a magnetometer to sense azimuth and an accelerometer to sense inclination. Signals from the sensor modules are typically received and processed in the control module of the downhole tool. The control module may incorporate specialized electronic components to digitize and store the sensor data. In addition, the control module may also direct the pulser modules to generate acoustic pulses within the flow of drilling fluid that contain information derived from the sensor signals. These pressure pulses are transmitted to the surface, where they are detected and decoded, thereby providing information to the drill operator.
It will be appreciated that the drilling environment subjects the sensors to a great deal of vibration. In addition, there is usually a monotonic increase in temperature with increasing depth. The rate of increase in temperature per unit depth in the earth is called the geothermal gradient. The geothermal gradient varies from one location to another, but it averages 25 to 30° C./km. Thus, at a well depth of 6 km, the temperature could be close to 200° C. Accordingly, the sensors and the other electronics on the BHA have to be able to withstand such high temperatures.
After the well has been drilled, additional measurements are made using sensors conveyed on a wireline or coiled tubing. These sensors are used for obtaining additional measurements of properties of the earth formation. With wireline measurements, the mechanical stress on the sensors and electronics due to drillstring vibrations is absent, but it is still necessary that they withstand the high operating temperatures downhole.
In addition to the inherently high temperatures downhole, the electronics themselves are a heat source. For example, the components of a typical MWD system (i.e., a magnetometer, accelerometer, solenoid driver, microprocessor, power supply and gamma scintillator) may generate over 20 watts of heat. Overheating frequently results in failure or reduced life expectancy for thermally exposed electronic components. For example, photo multiplier tubes, which are used in gamma scintillators and nuclear detectors for converting light energy from a scintillating crystal into electrical current, cannot operate above 175° C. Consequently, cooling of the electronic components is important.
Numerous methods have been used in the past for cooling of downhole equipment. U.S. Pat. No. 5,265,677 to Schultz discloses a downhole cooling system including a container holding a refrigerant. The cooling system also includes heat transfer elements for conducting refrigerant from the container in proximity to the electrical member so that a temperature adjacent the electrical member is less than ambient well bore temperature and preferably less than the maximum of the rated temperature operating range. The cooling system further includes a device for moving refrigerant from the container and through the heat transfer elements in response to pressure in the well bore. Another example of a refrigerant based cooling system is discloses in U.S. Pat. No. 6,769,487 to Hache.
Cooling systems based on refrigeration suffer from several drawbacks. These include their complexity. Compressor seals do not perform their function properly at elevated temperatures. In addition, they require a power source. The maximum coefficient of performance (COPmax) of a refrigerator is given by Tl/ΔT, where Tl is the temperature of the low temperature reservoir (such as the elctronics) and ΔT is the temperature differential. With increased temperature differential, the COPmax is reduced, so that more work is wasted in the refrigeration cycle. The wasted work appears in the system as additional heat! There is thus an inherent limit on the utility of refrigerant based cooling systems. It is to be noted that the COPmax is a theoretical upper limit, and any practical device typically achieves a small fraction of COPmax, typically 20-30%.
Thermoelectric coolers are also part of the prior art in the field. U.S. Pat. No. 4,375,157 to Boesen, includes thermoelectric coolers that are powered from the surface. The thermoelectric coolers transfer heat from the electronics area within a Dewar flask to the well fluid by means of a vapor phase heat transfer pipe. U.S. Pat. No. 5,931,000 and U.S. Pat. No. 6,134,892 to Turner et al. discloses a system in which thermoelectric cooling is used as part of a cascaded cooling system.
In the most general sense, thermoelectricity can be defined as the conversion of temperature differences to electricity and vice-versa. Two examples of thermoelectricity are the Peltier-Seebeck effect (thermocouples) and thermionic conversion (heating a material to release electrons). Seebeck formed a closed loop by joining the ends of two wires of dissimilar metals (a thermocouple circuit) and found that when the two junctions of the metal wires are at different temperatures, a voltage is created that is proportional to the temperature difference between the junctions. The Peltier effect is the reverse of the Seebeck effect. It corresponds to creation of a temperature difference from an applied voltage. Peltier found that when a current passes through a thermocouple, the temperature of one junction increases while the temperature of the other decreases, so that heat is transferred between junctions. The heat flow is proportional to the electrical current and the direction of heat flow is reversed when the current is reversed. Thermionic conversion is the generation of an electric current when electrons released by thermionic emission are collected. Thermionic emission is the ejection of electrons from a material when it is heated hot enough to raise some of the electron's energy above the binding energy (work function) of the material. It is the basis for a vacuum diode tube in which electrons are ejected from a heated anode are collected at a cathode. Thermoelectric cooling can be achieved through thermionic conversion. FIG. 5b is a schematic representation of such a thermionic cooler. A voltage source 205 is connected to a collector 201 and an emitter 207 of electrons 203. Under certain conditions, a temperature difference results due to heat 221 being extracted from the collector and the emitter is cooled. FIG. 5a is a circuit representation of a thermionic cooler. FIG. 6a is a representation of thermionic cooling as a diode. In this example, there is a small distance d of 1-10 μm between two electrodes and a potential difference (work function) ΔE between them. In order to get a significant number of electrons to jump over the barrier, the material must be heated and must have a low work function. The lowest work function materials are based on Alkali metals such as cesium, where the work function approaches 1 eV. Most metals have work functions in the 4-5 eV range. At 4-5 eV, significant emission does not occur until the cathode is hotter than 2000° K. Some metals melt before they emit electrons. Thoriated tungsten, which is used for cathode ray tubes, is heated to 1,950° K. There is not much need for forced cooling of objects at such high temperatures above ambient. The actual efficiency achieved with prior art thermionic cooling is typically 5-10% of the theoretical Carnot limit COPmax. A third example of thermoelectricity is thermotunneling in which electrons can quantum-mechanically tunnel from one unheated material to another when the distance between the two materials is small enough. Because it operates on a different principle, a quantum thermocooler can operate efficiently at the 80-200 C temperatures that are commonly found in the downhole environment. This is discussed below with reference to FIG. 6b. 
As noted above, Dewar flasks have been used in conjunction with thermoelectric coolers. To reduce the thermal load, tool designers have tried surrounding electronic components with thermal insulators or placed the electronics in a vacuum flask. Such attempts at thermal load reduction, while partially successful, have proven problematic in part because of heat conducted from outside the electronics chamber and into the electronics flask via the feed-through wires connected to the electronic components. Moreover, heat generated by the electronic components trapped inside of the flask also raises the ambient operating temperature. The term “electronic components” is intended to include electronic circuitry as well as sensors that operate on principles of electronics.
Typically, the electronic insulator flasks have utilized high thermal capacity materials to insulate the electronics to retard heat transfer from the bore hole into the tool and into the electronics chamber. Designers place insulators adjacent to the electronic components to retard the increase in temperature caused by heat entering the flask and heat generated within the flask by the electronics. The design goal is to keep the ambient temperature inside of the electronics chamber flask below the critical temperature at which electronic failure may occur. Designers seek to keep the temperature below critical for the duration of the logging run, which is usually less than 12 hours.
Electronic container flasks, unfortunately, take as long to cool down as they take to heat up. Thus, once the internal flask temperature exceeds the critical temperature for the electronics, it requires many hours to cool down before an electronics flask can be used again safely. Thus, there is a need to provide an electronics component cooling system that actually removes heat from the flask or electronics/sensor region without requiring extremely long cool down cycles which impede downhole operations. As discussed above, electronic cooling via thermoelectric and compressor cooling systems has been considered, however, neither have proven to be viable solutions.
U.S. Pat. No. 6,341,498 to DiFoggio teaches a cooling system in which an electronic component is cooled by using one or more containers of liquid and sorbent that transfer heat from the component to the fluid in the well bore. The electronic components are part of a downhole tool that may be on a drillstring through which a drilling fluid flows, a wireline, or coiled tubing. This cooling system comprises a housing adapted to be disposed in a wellbore, the sorption cooler comprising a water supply adjacent to a sensor or electronics to be cooled; a Dewar flask lined with phase change material surrounding the electronics/sensor and liquid supply; a vapor passage for transferring vapor from the water supply; and a sorbent in thermal contact with the housing for receiving and adsorbing the water vapor from the vapor passage and transferring the heat from the sorbed water vapor through the housing to the drilling fluid or well bore. The electronic circuits or sensors adjacent to the water supply are cooled by the evaporation of the liquid. While a major advance over earlier methods, the cooling capacity is limited by the amount of phase change material that is conveyed downhole.
Thus, there is a need for a cooling system that addresses the problems encountered in known systems discussed above. Consequently, it would be desirable to provide a rugged yet reliable system for effectively cooling the electronic circuits and sensors utilized that is suitable for use in a well bore. It is desirable to provide a cooling system that is capable of being used in an assembly of a drillstring or wireline.